A semiconductor detector substrate used for detection of x-rays and gamma rays may comprise cadmium zinc telluride (CdZnTe otherwise known as CZT) or cadmium telluride (CdTe). The amplitude of the electrical pulses derived from such detectors are indicative of the energy of the gamma rays absorbed by the detector substrate. Such semiconductor detector substrates comprise a plurality of detector cells (e.g., pixel cells) defined by an array of metal contacts on one side of the semiconductor detector substrate. The readout device can comprise a corresponding plurality of readout circuits each corresponding to each of the detector cells in the array. In the prior art, a semiconductor readout substrate is interconnected to the detector substrate with individual pixel cells being directly connected to their corresponding readout circuits by means of conductive bumps. Such a detector-readout assembly or module may then become part of a larger system used for creating images in two or more dimensions from x-rays or gamma rays being emitted by an object to be imaged. Alternately, the detector-readout assembly may be used singly, or in combination with other similar assemblies, to detect the presence of x- or gamma radiation photons and their energies.
Devices of this type have many important potential uses in biological and clinical imaging applications, environmental remediation systems, nuclear radioisotope security systems, and space satellites. In medical/biological applications, these array detectors have applications in planar imaging, SPECT imaging systems, and as surgical probes. Some possible applications are mammography, clinical cardiology, in vivo auto radiography, neuroscience studies, and lymphatic system imaging. In nuclear medicine, arrays of CZT detectors can create superior images of injected radiotracers, thus aiding in removal of cancerous tissue while minimizing damage to healthy tissue. They can also be used for medical applications involving the exposure of a patient to ionizing radiation. Such applications require high radiation absorption characteristics for the detector substrate of the imaging device. Such high radiation absorption characteristics can be provided by materials using high Z element, such as found in CdZnTe or CdTe. Furthermore, various medical applications require high spatial resolution. For example, mammography requires the ability to observe microcalcifications which can be under 100 microns or even under 50 microns in size. The stringent requirements imposed on imaging devices require the use of small resolution elements, or pixel cells, with a large array of such cells being needed to generate an image of a useful size.
Outside of biological and clinical uses, for environmental monitoring and remediation, as well as nuclear radioisotope security, gamma array detection can provide detailed information on radioisotopes present and their relative abundances. It also can be combined with an X-ray source to analyze the composition of non-radioactive isotopes through use of X-ray fluorescence, as for example, in examining the contents of a closed box or suitcase. In nuclear non-proliferation, the imaging of x-ray and gamma sources at a distance has the potential to detect illicit transport of radioactive materials. In astrophysics, CZT detector arrays are currently being employed in studies of distant gamma-burst sources.
An important step in the fabrication of such imaging devices is the interconnection of the semiconductor detector substrate array to the readout device and the subsequent interconnection of this assembly to other image processing electronics, electrical power and ground sources. This involves the electrical interconnection of the semiconductor detector substrate array cells to corresponding readout device cells in a one-to-one correspondence, and furthermore, electrical interconnection of the readout device to external electronics.
Typically, prior art imaging devices, known as hybrids, such as those described in U.S. Pat. No. 5,245,191, EP-A-0 571, 135, and EP-A-0 577 187, employ indium bumps for bump-bonding the semiconductor detector substrate directly to a semiconductor readout substrate. Generally in the prior art, indium bumps are grown using evaporation on the detector metal contacts that define the pixel cells and the corresponding readout device cells of a semiconductor readout substrate. Subsequently, the two different parts are brought together, aligned, and the corresponding bumps are merged. For indium bumps, a cold welding technique is achieved by heating the substrates at 70-120 degrees C. and applying mechanical pressure. For detectors comprising heat sensitive materials, such as CZT and CdTe, the use of indium bumps is advantageous in that the interconnection process can be carried out at temperatures below 120 degrees C.
U.S. Pat. No. 5,952,646, which is incorporated herein in its entirety, describes an alternate method in which low temperature tin-lead-based solder bumps, e.g. eutectic tin-lead-bismuth alloy (melt point 97 degrees C.), are employed instead of the more generally used indium bumps. The soldering of such bumps can also be accomplished at temperatures below 120 degrees C. The limitation of using such low-temperature solder is that the solder joints formed are relatively weak and subject to cracking and breaking. This can occur during normal use, as for example, when a detector assembly is subjected to thermal cycles in the operating environment, or when an assembly is dropped. Therefore, the use of low-temperature solders alone, as proposed in the prior art, is not practical for many applications.
Another issue is that creation of large detectors requires a package that allows detectors modules to be abutted together into large tiled arrays, without dead space in between detector modules. For example, U.S. Pat. No. 5,786,597, which is incorporated herein in its entirety, describes an alternative detector module configuration for such an abuttable detector module. In this patent, each detector module comprises a plurality of detection elements mounted to a circuit carrier, as shown in FIG. 1. The detection module comprises an integrated circuit mounted on a ceramic or plastic carrier 214. The circuit carrier 214 houses the readout ICs and passive components, and provides interconnections from the ICs to the detection elements 212 and to a module motherboard (not shown). The detection elements 212 are formed by an array of electrodes on the lower surface of the CZT detector 210. The prior art detection module 206 shown in FIG. 1 is assembled with thin plates positioned on both the top and bottom surfaces of the detector 210. The upper plate (not shown) provides a means for applying a bias voltage to the detection modules 206, insulates the bias voltage from the detector housing, and provides physical protection for the detector substrate. The upper plate is designed to allow the gamma rays emitted from the object 102 to penetrate the plate and to be absorbed in the detection elements 212. A lower plate 230 provides the means for connecting the detector elements 212 to the circuit carrier 214. The lower plate 230 includes a plurality of contact pads 232 that correspond in position to the positions of the detection elements 212. The plurality of contact pads 232 provide electrical connection for each detection element to a corresponding input contact pad on the top surface of the circuit carrier 214. The contact pads 232 are electrically isolated from each other. In the prior art, conductive epoxy or indium bump bonds are used to bond the electrodes of these detection elements 212 to contact pads on the lower plate 230 and the contacts of the lower plate to input contact pads of the circuit carrier 214. Thus, the detector inputs are connected to the readout ICs in the circuit carrier 214 via the upper surface of the carrier 214 and the lower plate 230. Other inputs and outputs are connected to the ICs via a plurality of pins 240 on the bottom surface of the circuit carrier 214. The plurality of pins 240 are designed to mate with insertion or socket connectors affixed to a motherboard. The configuration of the detection module allows the module 206 to be abutted on all four sides by other detectors on the motherboard. Therefore, such a prior art detection module 206 advantageously provides a modular element which can be combined in a number of ways with other detection modules 206 to produce a large imager tiled array having a desired configuration.
One problem with prior art. approaches that use of indium bump bonding to bond the various elements electrically is that the indium bump bonding process necessitates that the surfaces of the parts to be bonded be flat to very high tolerances. This is expensive to achieve and difficult to accomplish repeatedly in a production process. Also, if a conductive adhesive or indium bump bonding is used, a lower plate is required that is thick enough to mechanically isolate the semiconductor detector FIG. 1 210 from the carrier 214. Without such a lower plate, the stresses induced in the assembly's bonds may be capable of breaking such bonds and causing electrical failure of the element interconnects. Such stress can be caused by thermal cycling due to temperature fluctuations present under normal use, or due to mechanical shock, as when the assembly is dropped. However, the inclusion of a thick lower plate involves additional fabrication cost as well as increases the profile, or thickness, of the overall detector module. Low profile modules are desirable in many applications, e.g. modules employed in portable handheld gamma ray detection electronics.
Therefore there is a need to devise an improved method of bonding gamma ray array detectors, such as CZT and CdTe, to a readout device in such a way that it can withstand the stresses encountered in normal use. There is also a need to create lower cost abuttable detector modules having low profiles.